The present invention relates to a continuous rolling method for rolling blooms of steel or non-ferrous metal into billets as materials for various products or rolling said billets into various products and a continuous rolling mill for practicing the method.
Heretofore, continuous-cast blooms are normally used in rolling, for example, bar steel. In a blooming mill, a continuous-cast bloom is rolled into billets, reheated, and thereafter rolled and formed into various products in a steel bar mill or wire rod mill.
The rolling mill used heretofore in a blooming mill is normally a continuous rolling mill in which horizontal mills and vertical mills are arranged alternately. In this arrangement, both the horizontal and the vertical mills are driven both in the steel bar mills and the wire rod mills.
The term "horizontal mill" as used in the specification and claims is to be understood to mean a rolling mill of the type having a pair of work rolls disposed in parallel in the direction of the width of the rolled material to hold the rolled material between them from both sides and thereby reduce the rolled material in the thicknesswise direction. The term "vertical mill" as used herein and in the claims is to be understood to mean a rolling mill of the type having a pair of work rolls disposed vertically to the surface of the rolled material to hold the longitudinal edges of the rolled material between them and thereby reduce the rolled material in the widthwise direction. The expression "a rolling mill is driven" as used herein is to be understood to mean that the work rolls mentioned above are driven to rotate.
A vertical mill requires three times or more equipment cost than a horizontal mill of the same power because a work roll driving device is located in the upper portion of the mill housing. For the same reason, the vertical mill is more than five meters in height and, accordingly, the mill house is inevitably higher and longer. Therefore, vertical mills require much more costs than horizontal mills both in equipment proper and in building of their housings.
In order to overcome this disadvantage, the present applicant has proposed in Japanese Patent Public Disclosure No. 187203/83 Official Gazette (Patent Application No. 70208/82) the technical idea of making vertical mills undriven in a continuous rolling mill having horizontal mills and vertical mills arranged alternately. However, the technical idea of merely making the vertical mills undriven is not sufficient because the rolled material would buckle between the driven horizontal mills and downstream undriven vertical mills which makes continued rolling operation difficult. For this reason, the reduction of area in an undriven vertical mill is predetermined to be 66% or lower that of a driven horizontal mill on the upstream side. In such arrangement, the total quantity of thickness reduction by the horizontal mills becomes nearly twice the total quantity of width reduction by the vertical mills. Therefore, when a billet or product of square section is required, a material of rectangular section having a large flatness must be used because a material of square section cannot be used in a continuous rolling mill as described above.
On the other hand, requirements for the quality of materials for bar steel are very strict. In particular, decreasing both non-metallic inclusions and central segregation is important. Rolling of materials such as slabs is not allowed because it leads to increasing central segregation. Blooms widely used have generally sectional sizes from thickness 300 mm.times.width 300 mm to thickness 300 mm.times.width 400 mm. The technical art disclosed in the above-mentioned patent application is difficult to be applied to such blooms of square or nearly square sections.